The Brome mosaic virus (BMV) system is an excellent model for understanding the role of RNA recombination in plus-strand RNA viruses. Recently we have determined that Brome mosaic virus (BMV), a model plus-strand RNA virus, could replicate and recombine in Arabidopsis thaliana (Dzianott and Bujarski, Virology 2004). Our long term goal is to elucidate the role of the host genes in viral RNA recombination. The specific hypothesis is that the RNA interference (RNAi/PTGS) pathway participates in the generation of viral RNA recombinants. We base that hypothesis on the observations in Arabidopsis that: (i) the suppressor of RNAi/PTGS inhibited a type of RNA recombination that was supported by double-stranded structures; (ii) knocking-out an Argonaute gene inhibited non-homologous recombination in Arabidopsis; and (iii) homologous recombination was affected to a lesser extent than non-homologous recombination. Based on these data, the experimental focus of this application is on BMV RNA recombination in the Arabidopsis lines carrying the affected RNAi/PTGS pathway. The specific aims are to: 1. Characterize the role of DICER related host genes in recombination by using the Arabidopsis mutant lines. 2. Characterize the role of post-DICER related host genes in recombination by using the Arabidopsis mutant lines.